[Bug c++/58140] New: -Wnon-virtual-dtor shouldn't fire for classes declared final

[Bug c++/58140] New: -Wnon-virtual-dtor shouldn't fire for classes declared final

[tudorb at fb dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58140

            Bug ID: 58140
           Summary: -Wnon-virtual-dtor shouldn't fire for classes declared
                    final
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tudorb at fb dot com

Created attachment 30636
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30636&action=edit
Test case

In C++11, we can declare a class as "final" to indicate that it can't be
derived from. In that case, having a public non-virtual destructor is fine,
even if the class has virtual methods (no derived classes exist, so deleting an
instance via a pointer is always safe).

In the attached example, the warning should fire for NonFinalDerived, but not
for FinalDerived.

[Bug c++/58140] -Wnon-virtual-dtor shouldn't fire for classes declared final

[tudorb at fb dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58140

--- Comment #1 from Tudor Bosman <tudorb at fb dot com> ---
(Tested with gcc 4.7.1, compiled with -std=c++11 -Wnon-virtual-dtor

[Bug c++/58140] -Wnon-virtual-dtor shouldn't fire for classes declared final

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58140

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-08-12
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This should be pretty simple to fix, but why use -Wnon-virtual-dtor anyway,
when -Wdelete-non-virtual-dtor is more accurate and more useful?

[Bug c++/58140] -Wnon-virtual-dtor shouldn't fire for classes declared final

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58140

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Tudor Bosman from comment #0)
> In C++11, we can declare a class as "final" to indicate that it can't be
> derived from. In that case, having a public non-virtual destructor is fine,
> even if the class has virtual methods (no derived classes exist, so deleting
> an instance via a pointer is always safe).

N.B. this is only true if there's no base class with a public destructor, which
is true for your example, but not in general.

[Bug c++/58140] -Wnon-virtual-dtor shouldn't fire for classes declared final

[andrewjcg at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58140

Andrew Gallagher <andrewjcg at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrewjcg at gmail dot com

--- Comment #4 from Andrew Gallagher <andrewjcg at gmail dot com> ---
FWIW, I suppose one case where "-Wnon-virtual-dtor" is more useful than
"-Wdelete-non-virtual-dtor" is when the actual delete occurs in system
headers/libs.  This seems to be case when using std::unique_ptr, where the
actual "-Wdelete-non-virtual-dtor" warning is muted unless "-Wsystem-headers"
is used, which might not be desirable:

#include <memory>

class A {
public:
  virtual void a() {}
};

class B : public A {
public:
  virtual void b() {}
};

int main(int argc, char ** argv) {
  A *a = new B();
  delete a;  // triggers -Wdelete-non-virtual-dtor

  std::unique_ptr<A> p(new B());  // doesn't
}

[Bug c++/58140] -Wnon-virtual-dtor shouldn't fire for classes declared final

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58140

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That's PR 58876, which I intend to fix